The instant invention is directed to voltage reference circuits and, more specifically, to an improved Kelvin-connected buried zener diode voltage reference circuit.
The use of a zener diode as the reference element in a voltage reference circuit is well-known. It is also known in the prior art to provide such reference circuits with a subsurface breakdown zener diode; i.e., a buried zener. This type of diode, which is seen in U.S. Pat. No. 4,136,349 to Tsang, is more stable than conventional zener diodes making it ideal for precision circuits. Further, it is also well-known to provide a buried zener diode with a double-anode structure, or Kelvin-connection, as seen in FIG. 3 of U.S. Pat. No. 3,881,179 to Howard, Jr.
A Kelvin-connected buried zener diode has also been used in a voltage reference circuit as shown in FIG. 16 in the article "Circuit Techniques For Achieving High Speed-High Resolution A/D Conversion", IEEE Journal of Solid-State Circuits, Vol. SC-15, No. 6, pp. 1040-1050 (1980). In this circuit, one anode of the zener is fed with a bias current, while the other anode is used as a sense connection for error correction circuitry which compensates for both temperature and voltage variations in the diode. Finally, it is also known in the prior art to provide voltage reference circuits comprising a negative temperature coefficient diode-connected transistor in series with a positive temperature coefficient zener diode such that the overall temperature coefficient of the circuit is minimized. Such a structure is seen in U.S. Pat. No. 4,258,311 to Tokuda et al.
The prior art voltage reference circuits discussed above are reasonably efficient for non-precision applications, however, these circuits have inherent problems due to their design. For example, when a series combination of a buried zener and a forward-biased base-emitter diode are used as the voltage reference circuit, the inherent series resistance in the buried side of the zener affects the temperature sensitivity of the circuit. In particular, the temperature coefficient of the voltage drop across the series resistance cannot be readily compensated for by the base-emitter diode. The Kelvin-connected structure ameliorates this problem by separating the voltage drop from the voltage sense path thereby allowing the use of error correction circuitry, as described in the article referenced above. The use of correction circuitry to absorb and track the varying voltage difference between the Kelvin contacts, however, makes such a circuit prohibitively expensive. Moreover, such additional circuitry is inefficient when a single supply system is used; i.e., where the reference voltage is referenced to ground.
It is, therefore, desirable to provide a Kelvin-connected buried zener reference circuit which does not require extra compensation circuitry and which can be economically produced.